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Function-oriented Knowledge Base \ Electronic paper \ Improve image quality of paper-like display

Microencapsulation of electrophoretic composition improves electrophoretic display stability

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Introduction
Electrophoretic displays are paper-like non-emissive displays, which utilize the phenomenon of electrophoresis to achieve a contrasting image. A conventional electrophoretic display is bistable; its state persists after the activating electric field is removed. The state persists via the residual charge on the electrodes and van der Waals interactions between the electrophoretic particles and the walls of the electrophoretic cell. However, conventional electrophoretic displays are insufficiently stable because they are affected by agglomeration. Even when matching densities between the particles and the fluid, cohesive forces among the particles overcome dispersive forces and degrade the function of the display. Particle agglomerations respond less efficiently to the applied field, thereby increasing the switching time. The agglomeration results in a higher vulnerability to the action of gravity (limiting the usefulness of the display in arbitrary orientations). For example, if a display is oriented vertically, gravity can overcome the adhesion of a particle to the cell wall and cause the agglomerations to settle. It is necessary to improve the electrophoretic display’s stability.
 
Description
Using microencapsulation of electrophoretic components to improve the electrophoretic display’s stability is proposed. An electrophoretic display has a two-dimensional system. This system consists of a layer of transparent microcapsules that contain an electrophoretic composition of a dielectric liquid and a suspension of particles. These suspended particles visually contrast with the dielectric liquid and exhibit surface charges. The microcapsule layer is located between two visually transparent electrodes across which a potential difference may be applied. The potential difference applied across the two electrodes causes the particles within a capsule to migrate toward one of the electrodes. The microcapsules function in a manner similar to pixels, although they are not individually addressable. When the potential difference is removed, the particles agglomerate only within one microcapsule. The agglomeration effect is thus confined to a very small area that is sufficiently small to be individually unnoticeable. The effect of agglomeration on the function of the display in general becomes insignificant. Consequently, microencapsulation eliminates large-scale effects of agglomeration on an image. This improves the stability of the display. Therefore, microencapsulation eliminates the effects of agglomeration and improves the electrophoretic display’s stability.
 
Additional information
The amount of agglomerating particles is limited by one microcapsule, so the effects of diminished field responsiveness in the display and vulnerability to gravity are limited by the same amount of particles. The dimensions of the microcapsules range from 5 to 500 microns, and ideally from 25 to 250 microns.
 
Reference
US Patent 5930026; Link >>
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